LED lighting system

Abstract

Method and system are disclosed for compensating for color variations due to thermal differences in LED based lighting systems. The method and system involves characterizing the LEDs to determine what PWM (pulse-width modulation) is needed at various operating temperatures to achieve a desired resultant color. The characterization data is then stored in the microprocessor either in the form of a correction factor or as actual data. When an operating temperature that is different from a calibration temperature is detected, the characterization data is used to adjust the PWM of the LEDs to restore the LEDs to the desired resultant color.

1. A method of compensating for color variation due to temperature change in an LED based lighting system having at least one LED array, wherein same color LEDs are electrically connected to form LED channels, the method comprising:
 determining a pulse width needed for at least one LED channel to achieve a predetermined target color at at least a first and a second temperature;  
 calculating a pulse width correction factor for the at least one LED channel based on the pulse width needed to achieve the predetermined target color at the at least first and second temperatures;  
 correcting the pulse width for the at least one LED channel using the pulse width correction factor to achieve the predetermined target color.  
 
     
     
       2. The method according to  claim 1 , wherein the pulse width correction factor is calculated for every LED array in the LED based lighting system. 
     
     
       3. The method according to  claim 1 , wherein the pulse width correction factor is calculated for some of the LED arrays in the LED based lighting system and then applied to other LED arrays in the LED based lighting system. 
     
     
       4. The method according to  claim 1 , further comprising calibrating the LED channels, including (a) maximizing an intensity level of one of the LED channels, and (b) adjusting an intensity level of at least one other LED channel until the predetermined target color is achieved. 
     
     
       5. The method according to  claim 4 , wherein the calibrating step further includes repeating steps (a) and (b) after a predetermined amount of time. 
     
     
       6. The method according to  claim 1 , wherein the first operating temperature is a calibration temperature. 
     
     
       7. The method according to  claim 6 , wherein the second operating temperature is less than the first operating temperature. 
     
     
       8. The method according to  claim 6 , wherein the second operating temperature is greater than the first operating temperature. 
     
     
       9. The method according to  claim 4 , further comprising detecting an operating temperature of the at least one LED array, wherein the step of correcting is performed only if a difference between the calibration temperature and the operating temperature is above a predefined threshold value. 
     
     
       10. The method according to  claim 1 , wherein the target color is defined by chromaticity coordinates. 
     
     
       11. An LED based lighting system with thermal compensation capability, comprising:
 at least one LED array having same color LEDs electrically connected to form LED channels;  
 a microprocessor connected to and independently controlling each LED channel of the LED array to achieve a desired resultant color;  
 a temperature sensor connected to the microprocessor for detecting an operating temperature of the at least one LED array; and  
 an algorithm stored on the microprocessor and configured to cause the microprocessor to compensate for differences in the operating temperature of the at least one LED array.  
 
     
     
       12. The system of  claim 11 , wherein the algorithm causes microprocessor to compensate for differences in an operating temperature of the at least one LED array by causing the microprocessor to:
 receive the desired resultant color;  
 apply pulse-width modulation to each LED channel, the pulse-width modulation having a pulse width calibrated to achieve to the resultant color;  
 monitor an operating temperature for the at least one LED array;  
 calculate a temperature difference between the operating temperature and a calibration temperature; and  
 adjust the pulse width of the pulse-width modulation for at least one LED channel based on the temperature difference.  
 
     
     
       13. The system of  claim 12 , wherein the algorithm is further configured to cause the microprocessor to wait a predetermined amount of time for the LED array to stabilize before detecting an operating temperature. 
     
     
       14. The system of  claim 11 , wherein the temperature sensor is an on-board temperature sensor that is built into the microprocessor. 
     
     
       15. The system of  claim 11 , wherein the LED arrays are installed as a transportation cabin lighting system. 
     
     
       16. The system of  claim 12 , wherein the microprocessor adjusts the pulse width of the pulse-width modulation using a correction factor for each LED channel. 
     
     
       17. The system of  claim 16 , wherein the correction factor for each LED channel is calculated from actual data measured for the LED channel. 
     
     
       18. The system of  claim 16 , wherein the correction factor for each LED channel is estimated from data measured for one or more other LED channels. 
     
     
       19. The system of  claim 12 , wherein the microprocessor adjusts the pulse width of the pulse-width modulation using a look-up table. 
     
     
       20. A method of compensating for color variation due to temperature change in an LED based lighting system having at least one LED array, wherein same color LEDs are electrically connected to form LED channels, the method comprising:
 deriving a temperature compensation curve for the LED array;  
 calculating a correction factor for at least one LED channel based on temperature compensation curve; and  
 compensating for differences in an operating temperature for the LED array.  
 
     
     
       21. The method of  claim 20 , wherein the step of compensating includes correcting a pulse width for the at least one LED channel using the correction factor. 
     
     
       22. The method according to  claim 20 , wherein the compensation curve is a pulse width versus temperature curve, further comprising determining the slope of the pulse width versus temperature curve. 
     
     
       23. The method according to  claim 22 , wherein the slope of the pulse width versus temperature curve is determined by averaging data from a predetermined number of LED channels. 
     
     
       24. The method according to  claim 22 , wherein the pulse width versus temperature curve is determined based on at least two data points. 
     
     
       25. The method according to  claim 20 , wherein the step of compensation is performed only if a difference between an operating temperature and a calibration temperature is above a predefined threshold value. 
     
     
       26. A method of calibrating an LED based lighting system having at least one LED array, wherein same color LEDs are electrically connected to form LED channels, the method comprising:
 (a) stabilizing the LED channels;  
 (b) maximizing an intensity level of one of the LED channels;  
 (c) adjusting an intensity level of at least one other LED channel until a predetermined target color is achieved;  
 (d) waiting a predetermined amount of time;  
 (e) readjusting the intensity level of the at least one other LED channel until the predetermined target color is achieved again; and  
 (f) storing the intensity levels of the LED channels as calibration intensity levels.  
 
     
     
       27. The method according to  claim 26 , further comprising measuring an operating temperature of the LED channels after step (f) and using the measured operating temperature as a calibration temperature. 
     
     
       28. The method according to  claim 27 , further comprising repeating steps (d)-(f) until a difference in calibration temperature between a current iteration and a previous iteration is below a predetermined threshold. 
     
     
       29. The method according to  claim 26 , further comprising calculating a pulse width correction factor for the LED channels using the stored calibration intensity levels.